This invention relates to urea production and in particular to a urea synthesis process of high energy efficiency starting from ammonia and carbon dioxide.
In the known art, industrial urea production processes are based on the synthesis of ammonium carbamate at high temperature (for example 175.degree.-250.degree. C.) and pressure (for example 12-25 MPa) from ammonia and carbon dioxide in accordance with the exothermic reaction: EQU 2NH.sub.3 +CO.sub.2 .revreaction.NH.sub.2 COONH.sub.4
which is then dehydrated to urea in the same reaction zone and under the same conditions in accordance with the endothermic reaction: EQU NH.sub.2 COONH.sub.4 .revreaction.NH.sub.2 CONH.sub.2 +H.sub.2 O
which proceeds consecutively with the ammonium carbamate formation.
Whereas under the stated conditions the first reaction is very fast and is strongly shifted towards the right, the carbamate dehydration reaction is slower and only partly shifted towards the right. The degree of conversion of carbamate to urea depends on the stated operating conditions, on the residence time in the reactor and on the excess of ammonia over the stoichiometric ratio between the ammonia and the carbon dioxide.
To obtain high conversion to urea and limit the formation of harmful by-products such as biuret and its homologues, the NH.sub.3 /CO.sub.2 molar ratio maintained in the reaction zone in industrial processes varies between 2.5 and 5.
The effluent obtained from the synthesis zone consists substantially of a solution of urea, water, unconverted ammonium carbamate and free ammonia.
The free ammonia and the ammonium carbamate contained in said effluent must be separated and recycled to the synthesis section for total conversion to urea, so that substantially only the product urea and its stoichiometric water are discharged from the plant, in accordance with the overall equation: EQU 2NH.sub.3 +CO.sub.2 .revreaction.NH.sub.2 CONH.sub.2 +H.sub.2 O
according to which each mole of urea produced is accompanied by one mole of water, generated by the dehydration of the carbamate. In certain industrially successful processes, such as those of GB patent 2087381 in the name of Snamprogetti and U.S. Pat. No. 4,208,347 in the name of Montedison, an initial recovery of the unconverted carbamate is effected in a first decomposer operating under the same pressure as the synthesis reactor, to thermally decompose part of the unconverted ammonium carbamate to urea and release a part of the dissolved free ammonia, by heating the solution by heat transfer with a heating fluid, normally medium pressure steam, preferably in vertical heat exchangers in which the urea solution flows as a thin film to facilitate mass transfer between the phases.
The ammonium carbamate decomposition can optionally be facilitated by a gaseous ammonia stream fed into the bottom of the first decomposer. Alternatively, the ammonia contained in excess in the effluent can be used as a self-stripping agent.
In those processes comprising a first decomposition stage at the same pressure as the synthesis reaction, the vapour produced by the carbamate decomposition is generally recycled to the synthesis zone. This recycling can be conducted either directly in the gaseous phase to thermally sustain the reactor, or by separately recovering part of the heat of condensation of the gaseous phase produced in the decomposer to produce steam for use in other plant sections, and feeding the recycle stream as a mixed phase to the reactor.
A urea solution containing a reduced quantity of carbamate and an excess of free ammonia is obtained from this first decomposition stage.
It has been proposed, for example in European Patent 98396 in the name of Montedison, to follow the first carbamate decomposition stage with an excess ammonia removal stage, conducted at the same pressure as the synthesis zone, comprising stripping in countercurrent with a stream of carbon dioxide in a film heat exchanger, but with the simultaneous supply of heat on the shell side by condensing steam limited to the upper part of the tube bundle. The lower part of the tube bundle therefore operates under adiabatic conditions. From the description of this patent it emerges however that this adiabatic part of the residual free ammonia removal process results in a high residual carbamate content in the urea solution, hence highly penalizing the subsequent plant sections which then have to recover the carbamate under energy-unfavourable conditions, so considerably increasing energy consumption.
European patent 213669 in the name of Stamicarbon proposes adiabatically stripping the urea solution leaving the reaction zone with part of the feed carbon dioxide at the reaction pressure, but without preceding it by a first thermal decomposition stage. This treatment is limited only to a minor portion of the effluent (30-50%) whereas the major portion (50-70%) is fed to conventional stripping with carbon dioxide using an external heat supply.
In this process only the minor portion is fed to the subsequent medium pressure carbamate decomposition stage, its heat of condensation being recovered at a convenient temperature level only for this minor part, whereas the major part is directly fed to the low pressure thermal decomposition stage, where the heat of condensation of this major part is recovered at a temperature too low for its convenient use in the plant, and has to be disposed of with the cooling water, so energy-penalizing the overall process. This process is also difficult to implement in terms of correctly maintaining the division of the parallel streams which follow these separate paths.